1. Field of the Invention
The present invention relates to solid-state laser technology. More specifically, the present invention relates to cladding techniques and materials for suppression of parasitic oscillations in solid-state laser oscillators and amplifiers.
2. Description of the Related Art
Recent advances in high-energy diode-pumped solid-state lasers have facilitated extensive developments in the architecture of laser components such as laserable slabs and rods. Solid-state laser slabs typically-include a thin planar solid-state gain medium (core plate) that is encapsulated by a solid crystal cladding. The core plate, primarily having a rectangular cross-section, is a key laser component affecting beam quality at high energy levels. Commercial laser slabs are typically comprised of single crystals, such as Yb:YAG (ytterbium doped yttrium-aluminum-garnet) and Nd:YAG (neodymium doped yttrium-aluminum-garnet), or nanocrystalline transparent ceramics, such as Y3Al5O12. The core plate often includes two undoped input and output sections bonded to a doped central section. The bonded core is then structurally integrated with a peripheral crystal plate cladding. The plate cladding, having a step-index refractivity interface with the core, suppresses parasitic oscillations that otherwise extract energy from the core gain medium.
Parasitic oscillations reduce the efficiency of a solid-state laser system by establishing undesirable and uncontrolled paths of laser oscillations that extract energy from the system. During optical pumping, some of the excited atoms of the active lasing entity will spontaneously decay, resulting in the emission of photons at the frequency of the laser transition. As these photons traverse the solid-state lasing media, they become amplified. If the photons generated by spontaneous decay are emitted at angles greater than the critical angle for total internal reflection; the photons become trapped and will travel through the solid-state laser material by total internal reflection until they reach the edge of the solid-state material. At the edge, these amplified spontaneous emissions can be totally or partially reflected back into the solid-state laser material. If the signal gain achieved by these photons in traversing the solid-state material exceeds the reflection losses at the edge, the process can proceed indefinitely, resulting in the effect known as parasitic oscillation.
A number of methods are known which have been partially successful in reducing parasitic oscillations. Internal reflections can be avoided by roughening the outer surfaces of the laser slab. However, light scattering caused by the crystal roughening substantially reduces the projected slab efficiency. Wedged surfaces on the laser slabs can be designed to reduce parasitics due to; internal reflections on polished surfaces.
While this method provides a partially satisfactory solution, it rarely eliminates parasitics completely because it is difficult to design angled surfaces that will not allow any stray laser radiation to find a closed path within the lasing medium. In addition, the fabrication of precisely wedged core plates of high planarity from the hard-to-machine YAG is a time-consuming and expensive technology. Another prior art method for reducing parasitic oscillations includes depositing evanescent thick coatings having a lowered refractive index on the laser slab. The major technological difficulties associated with this approach are lattice and thermal (CTE) mismatches of the deposited (e.g., sapphire) and core (YAG) materials. This can also lead to energy leaks at the slab edges.
A prior technique for controlling parasitic oscillations is to attach absorptive edge claddings to the laser slab. The cladding is designed to absorb the accumulated spontaneous emissions instead of reflecting and scattering the emissions, thereby preventing the occurrence of parasitic oscillation. The properly designed cladding can also manage (sink) heat fluxes in preferable directions. The cladding, typically comprised of solid sapphire plates for a YAG core plate, is attached to the core plate using conventional diffusion bonding. Thermal and lattice mismatches, however, make the diffusion bonding of sapphire plate cladding with the YAG core plate difficult and lead to imperfect and low strength interface formation with strong propensity to delamination, thermal stresses highly localized at the slab ends, thermal stresses concentrated at reflectors, reduced thermal conductivity at the imperfect interface, and energy leaks. In addition, the machining of sapphire cladding is a time-consuming and expensive process. Thus, all these methods for reducing parasitic oscillations have been used with some degree of success but have not been found to be entirely satisfactory for certain current more demanding applications.
Hence, a need exists in the art for an improved system or method for reducing parasitic oscillations in solid-state laser components that is more effective and easier to fabricate than conventional approaches.